This invention relates in general to reduction and defibering machinery and methods for the paper and pulp industry. More specifically, it relates to an improved pulper construction and a method of operation for a pulper that (1) control blanketing of the rotor-stator by large sheets of stock material to be pulped and (2) rapidly and reliably break up this large sheet material.
Pulpers are used in the paper and pulp industry to reduce a stock material such as wood pulp into a watery slurry suitable for making paper. The stock material is added to a tank of water that feeds a pulper where the stock material is broken down into fibers of a suitable size and consistency to make the desired paper product.
U.S. Pat. No. 4,365,761 discloses a pulper that uses a rotor and stator operating at a close clearance (e.g., 0.010 inch) to defiber a variety of materials which had prior to that invention been impossible to defiber. These materials were termed "unconventional" or "difficult" and included cotton, hemp, flax, rag, leather, high wet strength papers, synthetic fibers, sheets of stock formed of fibrous materials bound by adhesives, and in particular, the high wet strength board known as "shoe board". The rotor and stator described in the '761 patent are configured to "acquire" and cut difficult materials with a scissors-like action at a size reduction interface having a truncated conical geometry. This interface is defined in part by a series of generally triangular segments, or "lobes" of the stator. Each lobe curves along an outer edge of a generally circular base and inclines inwardly. The inner surface of these lobes defines a conical, as opposed to a cylindrical, interface. An outer cutting edge of the blades, on the base of the rotor, define the inner boundary of this interface. Scissoring action occurs between these blade cutting edges and the leading edge of each triangular stator lobe. Once acquired and reduced to a sufficiently small size, the material is defibered in the attrition zone of the pulper between the lobes and the outer edges of the blades.
To date the '761 pulper, sold by Bolton-Emerson Americas, Inc. of Lawrence, Mass. under the registered trade designation "Tornado.RTM." is the only commercial pulper which can handle such materials. The Tornado.RTM. pulper is believed to be used to prepare the slurries that make about half of the paper currencies now in circulation throughout the world.
The energy input to the Tornado.RTM. pulper is used to reduce in size and defiber the stock material, to recirculate the flow of defibered stock back to the pulper (or, alternatively, to transfer stock downstream on a continuous basis), and to agitate the stock held in the tank using a toroidal flow. The pulper must also effectively deal with problems such as the tendency of some stock to float or to settle in the tank ("submergence"), plugging of the defibering mechanisms, and "slugging" due to the rapid introduction of a mass of difficult material to the acquisition and attrition zones of the rotor-stator pair.
While the '761 Tornado.RTM. rotor-stator design works well, it has difficulty in processing large slabs of sheet material such as reel broke and bales of "market" or "purchased" hard or soft wood pulp being recycled into paper. Reel broke is rewound (reel), off-specification printed paper, typically in ten to twenty foot lengths with a typical 40 inch diameter. A guillotine splits these reels lengthwise, and this split reel broke is added to the pulper tank. Depending on how it enters the tank, it can open into a large slab that covers the rotor. This blocks a central axial flow of water and stock 42 (FIG. 1) to the rotor-stator pair to limit, or substantially stop, the flow of stock material to the attrition zone of the rotor-stator pair for defibering. 500 to 550 pound bales of pulp, typically in three foot by four foot by one and a half foot stacks, are added to the pulper tank after outer wire wraps are cut off the bales. Large sheets of the freed pulp can then blanket the rotor, or slide off the rotor and sink to the bottom of the tank. Submerged baled pulp resting at the tank bottom is not processed, or is eventually processed, but slowly. In a continuous operation where slabs of reel broke, baled pulp or the like are continuously added to the tank, if these large sheet materials are not rapidly and reliably pulped, the slurry in the tank can become overly viscous.
The prior art "Tornado.RTM." pulper uses a smooth, curved nose cone, e.g., the one denoted by reference number 32 in the '761 patent. It covers several bolts securing the rotor to a drive shaft and it re-directs a central, axial in-flow into a flow that is radially outward to an acquisition zone where the blades on the periphery of the rotor interact with the surrounding stator lobes.
A seeming straight-forward solution to the blanketing problem, mounting a central projection on the cone to hold off the large sheet materials, in fact only spears or drills the sheet materials. The speared slab remains intact and blankets the rotor. Such a projection then adds to the problem in that it holds the sheet material in place over,the rotor. The use of non-central projecting members to control blanketing is contra-indicated by the tremendous forces acting on any member so situated. Other problems with such projections are increased power needs, increased turbulence, and the tendency of cotton and stringy stock materials to wrap around the projections.
It is therefore a principal object of this invention to provide a pulper nose cone construction and a method for operating a pulper that control blanketing of the pulper's rotor and stator by large sheet stock materials to be pulped.
Another principal object is to provide a nose cone construction and pulping method that not only control blanketing, but also rapidly and reliably break apart the sheet material.
Another object is to provide such a construction and method without altering the construction or mode of operation of known rotor-stator interfaces capable of handling difficult materials.
Still another object is to provide a construction and method of operation of a pulper that operates in a continuous or batch mode and greatly increases the throughput rate for such large sheet stock materials as compared to known such pulpers.
Yet another object is to provide a nose cone construction and method of pulper operation with the foregoing advantages that are durable and characterized by additional power requirements within the capacity of conventional drives for pulpers that can reduce and defiber difficult materials.